1. Field of the Invention
Embodiments of the present invention relate to a nitride-based light-emitting device and a method of manufacturing the same, and more particularly, to a nitride-based light-emitting device with an electrode structure designed to improve luminous efficiency and a method of manufacturing the same.
2. Description of the Related Art
An ohmic contact between n- and p-electrodes is of great importance in realizing a light-emitting device such as a light-emitting diode (LED) or laser diode (LD) using a gallium nitride (GaN) semiconductor. In particular, an ohmic contact structure between a p-cladding layer and an electrode is essential to the performance of a nitride-based light-emitting device.
GaN-based LEDs are classified into top-emitting LEDs (TLEDs) and flip-chip LEDs (FCLEDs). Since in commonly used TLEDs, light exits through a p-ohmic electrode layer in contact with a p-cladding layer, the TLEDs require the p-ohmic electrode layer with high light transmittance, low specific contact resistivity, and low sheet resistance. Because light is emitted through a transparent insulating substrate in a FCLED design, a p-ohmic electrode layer in contact with a p-cladding layer must have high light reflectivity, low specific contact resistivity, and low sheet resistance.
However, due to an inverse relationship that most electrode materials have between optical and electrical characteristics and poor electrical characteristics such as low current injection and current spreading in a p-cladding layer, it is difficult to develop a p-ohmic electrode layer suitable for a TLED or FLED structure.
A metallic thin film structure including a transition metal such as nickel (Ni) as its main component, i.e., an oxidized semi-transparent nickel (Ni)/gold (Au) thin film, is now widely used to form a p-ohmic electrode layer in a top-emitting light-emitting device. It has been reported that the Ni-based metallic thin film is annealed in an oxygen (O2) ambient to form a semi-transparent ohmic contact layer with specific contact resistivity of about 10−4 to 10−3 Ωcm2.
When the ohmic contact layer with low specific contact resistivity is annealed at temperature of 500 to 600° C. in an O2 ambient, a nickel oxide (NiO) that is a p-type semiconductor oxide is formed on the island-like Au layer and between the Au layer and a p-GaN, thereby reducing a Schottky barrier height (SBH) at the interface between the p-GaN and Ni. Thus, holes that are majority carriers can be easily injected into the surface of the p-GaN, thereby increasing effective carrier concentration near the surface of the p-GaN. Furthermore, annealing of the Ni/Au ohmic contact to the p-GaN results in disassociation of a Mg-H complex in GaN, which reactivates Mg dopants by increasing their concentration on the surface of GaN. As a result of reactivation, effective carrier concentration increases above 10 weight percent on the surface of the p-GaN, which causes tunneling conductance between the p-GaN and the electrode layer (NiO) thus obtaining ohmic conductance characteristics.
Due to their low light utilization efficiency, TLEDs using an oxidized semi-transparent Ni/Au electrode film are not suited for realizing large-capacity, high brightness light-emitting devices.
Flip-chip light-emitting devices using a highly reflective electrode material like silver (Ag), aluminum (Al), or rhodium (Rh) have recently been developed to realize large-capacity, high-brightness light-emitting devices. While temporarily providing high luminous efficiency, use of the highly reflective material with a low work function makes it difficult to form a p-ohmic contact electrode with low specific ohmic contact resistivity and high thermal stability. Consequently, it is difficult to provide a long life span and high reliability of a light-emitting device.
For example, Al cannot be used for an ohmic contact because it tends to form a Schottky (non-ohmic) contact at the Al/p-GaN interface due to the low work function and strong tendency to form a nitride during annealing. A g can be used to form an ohmic contact to a p-GaN but suffers from thermal instability and a poor mechanical adhesion with the p-GaN, thus making it difficult to ensure reliability during manufacturing and operation of the light-emitting device. While providing a higher work function and thermal stability than Al and Ag, Rh has lower reflectivity and degrades ohmic characteristics during annealing.
A recently proposed approach to forming a good p-type transparent ohmic contact layer for a TLED is to directly or indirectly stack a transparent conducting oxide (TCO) such as indium tin oxide (ITO) on a p-cladding layer. However, this approach also suffers limitations such as a short life time and low reliability of the device due to the ohmic contact structure with high specific contact resistivity.
Mensz et al., in electronics letters 33 (24) pp. 2066, proposed Ni/A and Ni/Ag as a p-ohmic contact layer structure for a FLED. However, the proposed structure has the problem of low performance due to non-oxidation of Ni.
Furthermore, Michael R. Krames et al., in U.S. Patent Application Publication No. 2002/0171087 A1, disclosed p-electrodes, oxidized Ni/Ag and Au/NiOx/Al. However, fabrication of the p-type reflective ohmic contact layer is complicated and the p-type reflective ohmic contact layer has a low luminous efficiency due to its high specific ohmic contact resistivity.
The formation of a high quality p-ohmic contact layer can also be of considerable importance for LDs. Unlike a LED requiring an electrode with both excellent electrical and optical characteristics, a LD needs an ohmic electrode with low ohmic resistance and high thermal stability and heat dissipation during its operation. However, with high specific contact resistivities and a large amount of heat generated during operation, many commonly-used ohmic electrodes for LDs degrade the life span and reliability of the device.
Meanwhile, in order to realize a high quality nitride-based light-emitting device, a high concentration p-type nitride-based semiconductor must be successfully grown.
Group II elements in the periodic table such as magnesium (Mg), zinc (Zn), and beryllium (Be) are commonly used as a dopant for a p-type nitride-based semiconductor. However, these metal dopants cannot serve their functions since they have a strong tendency to combine with hydrogen (H2) and form various types of complexes, which causes passivation. Thus, activation is needed to debond H2 from the complex so that the dopant can serve their function.
The most common activation technique is rapid thermal annealing (RTA) or furnace annealing at high temperature above 750° C. in a nitrogen (N2) ambient for 3 minutes. Since a compensation effect by a large amount of N2 vacancies formed on the surface of a p-type nitride-based semiconductor occurs during the activation, it is impossible to increase effective hole (major carrier) concentration on the p-type nitride-based semiconductor above 1018/cm2. Furthermore, because the p-type nitride-based semiconductor has a sheet resistance of above 104 Ω/cm2 due to its low hole concentration, it is difficult to form a good p-ohmic contact layer, thus hindering efficient hole injection and current spreading.
Furthermore, to achieve lower specific ohmic resistivity between a p-type semiconductor and an electrode, an electrode with a higher work function than the p-type semiconductor is needed. However, a p-type nitride-based semiconductor has the highest work function currently available.
Therefore, a p-type nitride-based ohmic electrode is fabricated by depositing a metal material such as Ni, Au, palladium (Pd), or platinum (Pt) in a single layer or multilayers and subsequently annealing the same to generate a large amount of Ga vacancies favorable for ohmic contact formation at an interface between the p-type nitride-based semiconductor and the electrode. However, the p-ohmic contact electrode thus fabricated has insufficient specific ohmic contact resistivity and optical characteristics to realize a high quality nitride-based light-emitting device.